The invention relates in general to output drivers, and more particularly, to a TTL output driver having an increased high level of the output signal.
The typical TTL output driver comprises a Darlington-coupled transistor pair coupled between a first power supply conductor, V.sub.CC, and the output of the driver, and a bipolar transistor coupled between the output of the driver and a second power supply conductor, the latter of which operates at ground potential. The Darlington-coupled transistor pair and the bipolar transistor are responsive to first and second complementary control signals applied at the bases thereof. In addition, a diode and resistor are serially coupled between V.sub.CC and the Darlington-coupled transistor pair for limiting the current flow therethrough and preventing reverse conduction between the output of the driver and V.sub.CC. Assuming the first and second control signals are high and low respectively, the Darlington-coupled transistor pair is rendered conductive and the collector-emitter conduction path of the bipolar transistor is high impedance, thereby allowing the output signal to be pulled in a positive direction towards V.sub.CC.
The industry has established a generally accepted standard for the minimum level of a TTL high signal, that being, approximately 2.4 volts with respect to ground potential. Given the uncertainty in component values associated with the variation in the manufacturing process, it is desirable to maximize the level of the high output signal for providing an adequate margin above the industry standard. However, this margin is difficult to increase in the conventional TTL output driver in view of the commonly known requirements of the TTL output driver, for example, the output drive current over temperature specification (3 milliamps at -55.degree. C.), which inherently opposes a reduction in the potential developed across the aforementioned series diode and resistor in the output conduction path, not to mention the potential across the Darlington transistor pair.
Hence there is a need for an improved TTL output driver for providing an increased margin for the high output signal level.